Hi Jessicamessica,
1.You would need a comma if your dialogue were declarative ("This is the reason," Ellen remarked.)
2.As Ellen was asking a question, a question mark takes the place of the comma ("'What's the matter?' Ellen asked.)
3.Just to complete the discussion, if your dialogue were exclamatory, an exclamation mark would take the place of the comma ("What's wrong with you!" Ellen shouted.)
I hope this helps.